Radiation-curable compositions comprising a cationically polymerizable compound and a photoinitiator for cationic polymerization are well-known in the industry and are used, for example, as radiation-curable paints, photoresists or for the production of three-dimensional articles by stereolithography. The photoinitiator for the cationic polymerization is formed in these compositions by a strong latent acid, i.e. a compound which undergoes a photoreaction on irradiation to form a strong acid, which then initiates the cationic polymerization.
In photohardenable cationically polymerizable compounds, the acid which is produced but is unable to cause significant gelation of the formulation, pose the greatest obstacle to compositional stability. Complications also occur when said radiation-curable compositions are used in practice, since the cationic polymerization commences prematurely, i.e. even before irradiation. This is generally due to premature formation of acids in the compositions. The undesired formation of acid can be due, for example, to decomposition of the photoinitiator for example owing to moisture, heat, unintentional exposure to light or scattered light, or by unintentional entrainment of acid. The undesirable formation of acid frequently causes such a large increase in the viscosity of the compositions that the composition becomes unuseable for its intended purpose.
An undesirable viscosity increase (viscosity destabilization problem) of this nature occurs frequently in the conventional production of three-dimensional articles by stereolithography using compositions based on a cationically polymerizable compound and a photoinitiator for cationic polymerization. In a stereolithographic process, as described in greater detail in U.S. Pat. No. 4,575,330, which is incorporated herein by reference, three-dimensional articles are built up in layers from the radiation-curable composition by first irradiating a layer of the composition imagewise. The composition is typically irradiated either simultaneously over the entire area or in a predetermined pattern (with raster or vectorial scanning) using a UV/VIS light source until the layer has solidified to a desired layer thickness in the irradiated areas. A new layer of the radiation-curable composition is then provided over the layer that has already been solidified. The new layer is similarly irradiated simultaneously over the entire area or in a predetermined pattern forming a second solidified layer adhering to the first.
This layering and irradiating operation is continued so that repeated covering of the previously solidified material with new layers of curable composition and subsequent irradiation of the new layer produces a three-dimensional article, also known as the "green part". The so-called "green part" is typically not fully cured, but is sufficiently solidified to withstand its own weight. The green part is removed from the bath containing the radiation-curable composition and may be post-cured, such as by the application of heat and/or further irradiation, to produce a final cured article or product.
After removal of the preform or green part, the stereolithography bath can be replenished with fresh curable composition and used for the production of an additional green part. It has been found that the cationically curable stereolithography baths, which, for economic reasons, are usually only replenished, exhibit an unacceptable increase in viscosity. An increase in viscosity is unacceptable due to the fact that the stereolithography part building parameters are originally determined for a specific properties of the material (e.g., narrow, specific viscosity range). As the viscosity gradually increases, new part building parameters must be continuously developed and optimized to achieve good part building. Unfortunately, the determination and optimization of stereolithography part building parameters is a long and costly process, and can be done only by highly specialized users.
Additionally, undesirable viscosity increase is detrimental to the building of articles having complex shapes. Complexly shaped articles can have, for example, narrow gaps, corners or internal cavities that are connected to the outside via a very small hole, from which high-viscosity material can not flow through to a sufficient extent.
A highly viscous or thixotropic composition also increases the time required for leveling of the top surface of the liquid composition in the bath. The increased time for leveling can significantly reduce the productivity of a stereolithographic device. Accordingly, improvements towards viscosity stabilization are of particular importance in the field of stereolithography.
In the past various resin stabilizers have been proposed. U.S. Pat. No. 3,721,617 by Watt proposes the use of various cyclic amide gelation inhibitors for epoxy resins. While these may be useful in some resins, it has been found that some of these cyclic amides, for example polyvinylpyrolidinone, significantly inhibit the polymerization of epoxy resins. Many bases are capable of neutralizing the acid generated as a result of thermal, hydrolytic or light activation. However, some bases are strong enough to cause reaction or polymerization (work as catalysts) and therefore are not useful as viscosity stabilizers.
The publication of a DuPont international patent application, WO96/41238 describes the use of a viscosity stabilizer having limited solubility in the composition and having a density which is different from that of the composition wherein, the stabilizer is a salt of a metal of Group IA, a metal of group IIA, ammonia or a substituted ammonia and a weak acid and wherein the stabilizer in the composition is present in an amount that is in excess of its solubility. The concentration of the stabilizer in the formulation is maintained by the presence of the salt in excess of its limited solubility. Preferred stabilizers, as shown therein, are salts of metals of Group IA and weak inorganic acids.
U.S. Pat. No. 5,073,476 mentions that in order to increase the resin capacity to be stored in the dark, the curable compositions can contain weak organic bases such as nitrites, amides, ureas. In order to prevent premature reaction caused by unintentional exposure, small amounts of UV absorbers and/or organic dyes can be added.
It is known that the addition of filler material, such as inorganic materials, ceramics, composites, metallic filler, organic polymeric, glass, thermoplastics, silica beads, etc. to radiation-curable compositions improves the majority of the mechanical and thermomechanical properties of resulting cured articles. The filler material may be either acidic or basic or neutral depending on the surface characteristics of the filler. The incorporation of a filler material or a mixture of fillers in radiation-curable compositions for use in stereolithography systems usually introduces viscosity destabilization problems in the overall composition.
The present invention overcomes the highly undesirable problems associated with viscosity destabilization (viscosity increase), particularly in filled compositions for use in stereolithographic systems.